255 Extra Lives
Whenever you see a tall springy pole (that you can vault yourself off of) vault off of it and land on the top of the pole. If done correctly, you'll bounce 10 times on the pole, and gain 1 extra life for each bounce. After that, die and repeat the process.
Note: The maximum amount you can get is 255 lives. If you go over 255, you will be taken back to 1 Life.